I upgraded my iMac (model 14,2) from Mac OS X El Capitan to macOS Sierra yesterday.
The update went smoothly except that once the update finished and I logged in, the volume icon in the menu bar was greyed out. I immediately opened settings, assuming the update had somehow deselected my default audio-out device (a pair of external speakers in this case). However, when I went to the sound menu I was surprised to see "No output device found".
I've looked around quite a bit and haven't found anyone else with this issue yet. I became so desperate to get my audio back that I even tried installing the 3rd party "Voodoo" audio driver, however this only gave me the option for "Digital-out (HDMI)" which is of no use to me.
One thing this did clarify however is that somehow during the upgrade my AppleHDA driver got deleted and never replaced, or installing an 3rd party driver wouldn't have added any output devices to the list (albeit greyed out and entirely dysfunctional ones). So at this point I am at a loss, I don't know how to replace the driver, nor where or how I would install it.
I've browsed the
/System/Library/Extensions/ directory to discover I have both
AppleHDA892.kext, which makes the situation more confusing as the system possesses both drivers which should be required to recognize audio output devices as well as playback sound.
My setup is as follows: iMac, macOS Sierra (10.12), 27-inch (Model 14,2).